Neurofeedback theta/beta ratio training. The TBR Neurofeedback signal will be given by means of a NeXus-4 amplifier and recording system (Mindmedia, The Netherlands). One Ag/AgCl disposable electrode will be applied on the participant's scalp between locations Cz and FCz. For a high-quality and accurate EEG measurement, 16 extra in-cap electrodes (BioSemi, The Netherlands) will be added on locations F3, Fz, F4, C3, Cz, C4, P3, Pz, P4, T7, TP7, T8, TP8, O1, Oz and O2) during every session. Each session will consist of a four-minute resting state measurement, followed by a 25 minute measurement/Neurofeedback training or placebo training. The thresholds will be set 20% above (beta) or below (theta) the participant's amplitude, calculated and adjusted automatically every 15 seconds. During the training, theta and beta activity will be visualized in separate bar graphs on the screen and participants will be instructed to "try to move down the theta activity and move up the beta activity below or above the threshold". No instructions about how to influence their activity will be given. When the EEG theta power goes below the threshold, and the beta power above its threshold simultaneously, the participant will be rewarded by the continuation of a video. If they fail to reach this point, the video will be paused.